Richard, I'm not convinced that Royal Mail can pursue any claim against you, as the possibility of the item subsequently turning up much later, and being delivered, is not (as far as I can see) in their terms, conditions and exclusions of claim for loss. They define loss as follows:-
There is no further qualification to this (as far as I can see). Hence, in the absence of such provision, the item surely remains still legally "lost". The "evidence to the contrary to demonstrate that the item has not been lost" surely only applies up to and including the end of that tenth day. If it doesn't exist then, which it didn't, it never can.
link to RM loss/compensation rules